Directional controlled drilling arises from the early practices of using either a whipstock (wedge) set within a borehole to force a hole to deviate from a known trajectory, or the use of a jetting bit. Both are described in some detail in Applied Drilling Engineering, Society of Petroleum Engineers Textbook Series, Vol. 2, Chapter 8, Adam T. Bourgoyne Jr., Keith K. Millheim, Martin E. Chenevert & F. S. Young, Jr., 1991. The jetting system typically involves the use of a two-cone roller bit with a single stabilizer and a large jetting bit. When a directional adjustment is required, the drilling is interrupted and the large jet is held in the direction in which the deviation is required so that the jet erodes preferentially in that direction. Rotary drilling can resume after the desired directional change has been effected.
More recently most directional drilling has been undertaken by the use of down-hole mud motors. Turbine and positive displacement motors have been used with the latter being in more common use. Down-hole motors operate by converting energy extracted from the drilling fluid forced down the drill string and through the motor. This energy is converted into rotary motion which is used to rotate a drill bit that cuts the rock ahead of the tool. Directional change is effected by the use of a bottom hole assembly which includes a bent housing either behind or in front of the motor so that the bit does not drill straight ahead, but rather drills ahead and off to the side. This bottom hole assembly may be supported within the borehole by a series of stabilizers which assist the angle building capability of the assembly.
The bottom hole assembly so described tends to build an angle rather than drill straight ahead. Such a tendency can be halted in some drilling systems by rotating the entire drill string and bottom hole assembly so that on average the system drills straight ahead. A more common practice is to undertake repeated directional changes to the borehole trajectory by turning the rod string and hence the tool face angle. Alternatively, as is the case in coiled tubing drilling where the drill string cannot be rotated, the tool face is adjusted by incremental moves associated with fluid pressure pulses which relocate the tool at varying tool face angles. By changing the direction at which the bottom hole assembly tends to build an angle, many changes to the trajectory can be achieved. The borehole is seldom aligned in its intended direction but follows a snaking path about the planned direction. One of the consequences of this system of drilling is that the drill string is, by reason of the many changes in direction of the borehole, subject to much higher friction and stress levels. This is described in more detail in the publication Optimisation of Long Hole Drilling Equipment, Australian Mineral Industries Research Association, Melbourne, Ian Gray, March 1994. A consequence of the friction and stress is that the length of borehole is limited.
The basis for changing the direction in which drilling assemblies currently drill includes survey information measured near the bit, combined with a knowledge of the total distance drilled, and knowledge of the formation. The survey information normally provides information on the direction tangential to the survey tool located in the drill rods within the borehole. This information can be integrated with respect to the linear dimension of the borehole to arrive at the coordinates for the borehole. The formation position is either detected by prior drilling and geophysics or by geosteering equipment. The latter may comprise geophysical and drilling sensors to detect the nature of the material which is being drilled, or which are located at some distance from the drill string. The nature of the material being drilled is most likely to be detected using a torque and thrust sensor within the drill string, short focused gamma-gamma probes or resistivity probes. Alternatively, formation types may be detected at a greater distance by long spaced resistivity tools. On the basis of the information about the formation, the drilling direction is adjusted to keep it to near an optimal path.
The logical process of such adjustments is for the drilling to proceed upon an initial direction with an estimated rate of directional change. After some drilling, survey and/or geosteering information is obtained from down-hole sensors and is then transmitted upwardly to the borehole collar or wellhead. This transmission may be by withdrawal of the survey tool containing the information by wireline, by transmission up a cable or by using pressure pulses developed in the drilling fluid by solenoid or other valves which operate to partially restrict drilling fluid flow through a mud pulser section of the geosteering tool. An operator then interprets such information and adjusts the trajectory of the borehole accordingly. Normally, this would be achieved by changing the tool face angle and then continue drilling. This process is interactive, with the system being critically dependent on information flow from the down-hole tools to the operator. It is also highly dependent on the ability of the operator to interpret the information and accurately adjust the tool face angle accordingly. This is not a simple exercise when the likelihood exists for long drill strings to wind up several rotations between the bottom hole assembly and the drill rig at the surface.
An alternative to positive displacement motors and turbines for directional drilling is the use of fluid jets to erode a potential path. A well established system for the use of this equipment has been described above. There has also been a significant amount of interest in alternative drilling strategies using fluid jets to do all the cutting or to use them to assist modified conventional rotary drill bits. This work is well summarized in the publication entitled Water Jet/Jet Assisted Cutting and Drilling, IEA Coal Research, London, Peter A. Wood, 1987. With this technique it can be seen that fluid jets can be used to effectively cut coal and some rocks by impact and the action of high pressure fluid in the cracks.
The publication entitled Development of a High Pressure Waterjet Drilling System for Coalseams, thesis submitted in partial fulfillment for the degree of Masters of Engineering Science, Department of Mining and Metallurgical Engineering, University of Queensland, by Paul Kennerly, January 1990, describes the use of rotating heads producing fluid jets which are driven by reaction to the emitted jet streams. Pressures used in this work were of the order of 500-700 bar. In addition to forward facing cutters there are also rearward facing jets which are called retrojets. These rearward facing jets were introduced originally to supply additional flushing fluid to the borehole. The reactive thrust that they provided however was adequate to draw the EW rod drill string (1 3/8" outside and 7/8" inside diameter steel tube) into the borehole, and subsequently the steel drill rod string was dispensed with and drilling was accomplished using a flexible assembly. This consisted of a rotating nozzle, retro-jet jet assembly, ten meters of steel pipe followed by a hydraulic hose which was drawn into the borehole as part of the drill string.
The publication entitled Development of a Coalseam Water Jet Longhole Drill, a thesis submitted in partial fulfillment for the degree of Doctor of Philosophy, Department of Mining and Metallurgical Engineering, University of Queensland, by Paul Kennerly, July 1994, describes a further development of the fluid jet drilling system. In the final form reported herein, the drilling was accomplished using a rotating nozzle which was rotated by the reaction to angled forward facing jets. Behind these and on the same rotating nozzle were lateral facing reaming jets. This nozzle was contained within a shroud for its protection. Behind the shroud and nozzle either a bent drill sub and retro-jet unit were installed in that order or with the retro-jet unit ahead of the bent sub.
Directional control was achieved as in down-hole motor drilling by changing the tool face angle of the bent drill sub so that drilling would preferentially take place in the direction in which the sub was pointing.
One of the problems associated with pure fluid jet drilling is the comparative ease and difficulty with which soft and hard materials are cut. The Kennerly thesis reports that an acute angle intersection with a stone band within a coal seam led to the hole narrowing until the drilling apparatus jammed in the hole.
The potential exists to overcome this problem by introducing a drill bit with a reaming or cutting capability so that hard materials may be cut and so that the tendency for the drillhole to be deflected by hard and soft boundaries is reduced.
Such bit assisted fluid jet cutting is summarized in the Wood publication (pp 32 & 40). The publication Water-Jet Assisted Drilling of Small Diameter Rock Bolt Holes, National Energy Research, Development and Demonstration Program, End of Grant Report No. 598, Department of Resources and Energy, Canberra, Australia, D. A. Clark and T. Sharkey, 1985, describes the effectiveness of fluid jet assistance in reducing bit wear.
More recently the publications, In-seam Drilling Researchers' Meeting, CMTE, Brisbane, John Hanes, Apr. 23, 1996, and Presentation On Water Jet Assisted Rotary Drilling, Centre for Mining Equipment and Technology, Brisbane, Australia, Paul Dunn, May 23-24, 1996, referred to the use of fluid jet assisted drilling in coal. This described the use of an 80 mm drill bit being used in rotary drilling in a seam through coal with fluid jet assistance at 40 MPa and 20 MPa. The fluid jets appeared to reduce the bit thrust to a negligible level with the higher fluid pressures. The total distance reached was 250 m.
Another application of fluid jet drilling is described in the publication Data Acquisition, and Control While Drilling With Horizontal Water-Jet Drilling Systems, International Technical Meeting by the Petroleum Society of CIM, Calgary, Canada, Paper No. CIM/SPE 90-127, Wade Dickinson et al., Jun. 10-13, 1990, and in The Ultrashort-Radius Radial System, SPE Drilling Engineering, SPE Paper No. 14804, September 1989, Wade Dickinson et al., 1989. In these papers reference is made to the use of fluid jets to drill directionally controlled boreholes. The ultrashort-radius system employed the use of side thruster fluid jets to change the direction of the main fluid jet used to drill the hole. The larger system employed the use of a 4.5 inch diameter drilling system which uses a module that seats into the inner end of the drill string. This module is held on a wireline and contains several obliquely angled nozzles designed to erode in preferential paths. In both of these systems the directional control jets are operated by a wireline from the surface through the use of solenoid valves. Both systems refer to fluid pressures of 690 bar.
Directional control has been achieved in drilling without control from the surface. Deutsche Montan Technologie (DMT) described in the Automatic Directional Drilling System ZBE 3000, Deutshe Montan Technologie, (Internal technical publication), that a system was produced which uses rotary drilling to advance a borehole. Behind the bit was installed an electronic package which senses whether the borehole is out of vertical alignment. This controls pistons which press on the borehole annulus, forcing the drill string back into line.
A device similar in concept to that of DMT is a vertical drilling guidance system, but using a down-hole mud motor is described in Offshore Application of a Novel Technology for Drilling Vertical Boreholes, SPE Drilling & Completions, SPE Paper No. 28724, P. E. Foster and A. Aitken, March 1996.
Another application of directional drilling in which control decisions are made in the borehole is sketchily described in Automated Guidance Systems for Directional Drilling and Coiled Tubing Drilling, presented to the 1st European Coiled Tubing Roundtable, Aberdeen, Andrew Tugwell, Oct. 18-19, 1994. This system developed by Cambridge Radiation Technology uses some directional sensor/geosteering sensor technology to discern deviations from the planned well path. Corrections in direction are made by rotating a joint above the motor using a hydraulic servo system. The paper is somewhat confusing in that it also refers to a multi-cable system extended to the surface with control being conducted at the surface.
Differential stacking is a factor which influences all drilling where the mud pressure exceeds the formation pressure and particularly in cases where the drill string is not rotated or vibrated.